This invention relates generally to seismic exploration and more particularly to a field quality control system and real-time method for analyzing and adjusting the performance of swept frequency vibrators for use in seismic exploration.
The general method of seismic exploration using swept frequency vibrators is well known. Briefly stated, the method comprises the steps of imparting seismic energy into the earth with the swept frequency vibrator at a first point and recording the earth's response thereto with seismic wave detectors at other separate points on the earth's surface. The earth's response is recorded as an electric signal. By crosscorrelating a pilot sweep signal (which is used to control the vibrator) with the electric signals produced by each seismic wave detector, a number of traces representative of the earth's subterranean formations can be formed.
Due to the relatively low seismic energy produced by swept frequency vibrators, as well as to cancel a portion of the noise generated by the vibrators, it is customary to employ simultaneously several vibrators deployed in a pattern on the earth's surface to impart seismic energy and then subsequently to composite the individual recordings obtained from a number of adjacent patterns. Typically, four vibrators will simultaneously generate and impart seismic energy in a pattern and twenty or more of such patterns will be used to produce seismic signals which are composited to form a single vibrator point recording. Simultaneous use of several swept frequency vibrators requires synchronization so that the individual signals will reinforce. If during any one cycle of imparting seismic energy, the vibrators are not synchronized, (i.e., they produce different phase, amplitude and/or frequency seismic energy) destructive cancelation can degrade the received signals producing noisy traces in which desired reflection signals are obscured. Alternatively, a single vibrator can be employed to impart seismic energy into the earth at a plurality of locations and the resulting recorded signals can be summed to form a single vibrator point recording. Consequently, it is necessary that the seismic energy imparted by the single vibrator at different locations also be synchronized to ensure reinforcement of the signals during the summation process.
When seismic data are collected using swept frequency vibrators, it is crucial for the subsequent processing of the recorded seismic data that one have accurate knowledge of the nature of the signal imparted by the swept frequency vibrator. The swept frequency vibrators used in seismic prospecting produce sinusoidal, swept frequency signals whose amplitude and phase are dependent upon the swept frequency vibrator's electromechanical characteristics and the complex impedance of the earth beneath its baseplate. As a matter of practice, the signal imparted by the swept frequency vibrator is assumed to be substantially identical to a phase-shifted/timedelayed version of the pilot sweep signal which is used to control a swept frequency vibrator. By crosscorrelating the recorded seismic data with the pilot sweep signal, reflected seismic energy can be recovered and interpretations of the earth's subsurface geological structures can be made. Hence, the failure of the signal imparted by the swept frequency vibrator to track the pilot sweep signal in phase and frequency can result in serious errors in processing the recorded seismic data and consequently produce a false picture of or obscure the earth's subsurface geological structures.
Traditional vibrator quality control practices have required that a "similarity" test be run daily prior to commencing seismic data acquisitions to verify vibrator performance. The similarity test includes a comparison of selected vibrator output signals with the pilot sweep signal and/or output signals from other vibrators. The selected vibrator output signals can include baseplate velocity and reaction mass acceleration obtained from sensors integrally mounted with the swept frequency vibrator. After the similarity test has been conducted, generally there is no further check of vibrator performance during that day.
Until recently, vibrator performance in the field has been evaluated based on separate recordings and displays of vibrator output signals and pilot sweep signals. Typically, such recordings and displays comprise camera records which provide a very poor method for obtaining quantitative values of the vibrator output signals. Additionally, it is very difficult to analyze for more detailed information such as phase relationships of the output signals with the pilot sweep signal controlling the swept frequency vibrator.
Vibrator control units mounted with swept frequency vibrators have been developed to synchronize the velocity component of the vibrator baseplate with the pilot sweep signal. Such vibrator control units simultaneously monitor both the pilot sweep signal controlling the vibrator and the vibrator baseplate velocity or some other aspect of vibrator performance. The vibrator control units develop feedback signals to control the electromechanical actuator of the vibrator so as to phase-lock the pilot sweep signal and the baseplate velocity. Similar vibrator control units mounted with swept frequency vibrator have been provided to control groundforce amplitude and phase. The Sallas U.S. Pat. No. 4,637,002 is exemplary of such vibrator control units. However, such vibrator control units all suffer common limitations in that they provide no means for ascertaining vibrator operating characteristics independent of the vibrator's own sensing system, they provide no display to compare various aspects of the vibrator performance, and they fail to monitor sufficient vibrator operating characteristics to fully analyze and adjust the performance of the vibrator. Rather, performance is often judged solely on the basis of differences in the initiation (start) time and phase and it is mistakenly assumed that the amplitude of the baseplate will be uniform over the sweep bandwidth. Baseplate signal reproducibility, amplitude and distortion are equally important but less frequently used quality control criteria. Heretofore, these parameters were seldom specified or tested because they required time-consuming computer analysis of recorded vibrator output signals.
More recently, Pelton Company, Inc. has developed a system for monitoring vibrator performance. This system receives both the pilot sweep signal and an output signal from the vibrator and produces separate numeric printouts representing either frequency, relative amplitude or phase as a function of selected time intervals. This method is an advance in the testing of the swept frequency vibrators in the field; however, it lacks the ability to provide independent, calibrated measurements of swept frequency vibrator system performance. Specifically, it does not provide for verifying the correctness of the vibrator output signal used. This shortcoming becomes an even more important discrepancy when a groundforce signal is used to control the swept frequency vibrator. Additionally, the Pelton system fails to provide simultaneous analog displays of the pilot sweep signal and various output signals of the vibrator; rather, the Pelton system prints numeric values of the pilot sweep signal amplitude; vibrator output signal amplitude and phase; and the difference in phase of the pilot sweep signal and the vibrator output signal.
Texas Instruments has also developed a field system for quality control of swept frequency vibrators. This system provides more sophisticated processing of the pilot sweep signal and output signals of the vibrator. The Texas Instruments system cross compares any two sweep traces of the vibrator and outputs them to a plotter. But for the displays of two such sweep traces, it is understood that the Texas Instruments system merely displays a single representation of one of the following: auto correlation of the pilot sweep signal; crosscorrelation of the pilot sweep signal with an output signal of the vibrator; power spectrum of the pilot sweep signal; power spectrum of the output signal of the vibrator; power spectral difference between the pilot sweep signal and output signal; and a harmonic distortion plot. The Texas Instruments system also produces a numeric listing of power spectral differences as well as instantaneous frequency and phase difference versus time.
Others have developed computer programs designed to remotely analyze vibrator performance. The analysis associated with these programs has two serious shortcomings. First, as with the previous field devices, independent and calibrated measurements of the system performance are not available as input to the programs. Second, although the analyses performed by these programs are generally more detailed than that provided by field units, the remote location of the host computer is such that real-time field analysis and adjustment of vibrator performance is not possible.
The present invention is directed to a field quality control system for analyzing and adjusting in substantially real-time the performance of swept frequency vibrators which overcomes the limitations of existing systems. Specifically, the system includes independent calibrated accelerometers which are detachably mounted with the reaction mass and baseplate of the vibrator. The signals generated by such accelerometers are monitored and processed in real-time on location to provide immediate results of vibrator performance. This real-time field analysis is of extreme value since it allows monitoring the vibrator performance as adjustments are made. Thus, vibrators can be interactively adjusted and analyzed until performance specifications are achieved.
The system can display simultaneously and interactively two or more analysis characteristics representing the forces acting on the vibrator mass and baseplate, the groundforce as well as baseplate velocity and pilot sweep signals. These analysis characteristics can be displayed in their raw form, as crosscorrelated wavelet data, amplitude spectra data, phase spectra data or harmonic distortion. The displays can be scaled and/or windowed interactively in real-time. The analysis characteristics can be overlayed, archived for later recall, or spooled to a hardcopy device for documentation. The vibrator hold-down force can also be overlayed on the amplitude spectra data to indicate the limits of decoupling. Additionally, the phase error tolerance can be overlayed onto the phase spectra data, indicating acceptance levels of phase-locking of the vibrators.
The quality control system is also of considerable value in establishing operational parameters for swept frequency vibrators for a particular site location. In such cases, the swept frequency vibrator's performance is monitored and adjusted on site for optimized performance at a particular site location. Further, the system can provide useful diagnosis for correcting vibrator deficiencies. Moreover, for diagnostics not apparent from normal analysis, a plurality of vibrator deficiencies can be simulated with the system and identified through such analysis rather than through physically interchanging components of the swept frequency vibrator and observing the resulting performance to see if the problem has been corrected.